Metal fluoride optics, such as calcium fluoride optics, frequently include a coating deposited on the surface of the optic. The coating serves as a protective layer that prevents fluorine loss that occurs when the optic is exposed to radiation in high fluence laser applications. The fluorine loss causes degradation of the optic which ultimately leads to the failure of the optic and the need to replace the failed optic.
During manufacture of the coated optics, and after they are used, the coated optics are frequently cleaned using a solvent wipe such as, for example, a methanol wipe. Wiping is a mechanical technique for removing particulate matter from the surface of the coating. However, removal of particulate matter using such technique causes particles to be dragged across the surface of the coating, which can damage the coating. The use of a solvent wipe to clean coated optics has been observed to cause blistering, crazing and delamination of the coating. As used herein, the term “blistering” refers to dome shaped projections in the coating and local losses of adhesion of the coating which causes lifting of the coating from the underlying optic surface. Additionally, as used herein, the term “crazing” refers to cracks that penetrate into the coating. Also as used herein, the term “delamination” refers to loss of adhesion of the coating to the underlying optic surface, or to loss of adhesion between coating layers in coatings that include more than one layer.
Once damage such as blistering, crazing and/or delamination occurs, the coating's performance as a protective layer is reduced, and the adhesion of the coating to the surface of the optic is weakened. As a result, when the coating is damaged, fluorine loss in the optic, which the coating is meant to prevent, is observed during exposure to radiation in high fluence laser applications, and accelerated failure of the optic occurs.